1. Field of the Invention
The present invention relates to an electronic signal compression apparatus for compressing a composite input signal having a large dynamic range into a smaller dynamic range output signal such that only the large signal components, which in received radar pulse signals represent clutter, are selectively compressed while small signals, which represent desired target information, retain their original amplitude.
2. Description of the Prior Art
Linear dynamic range is an important parameter in modern radar and other types of receivers. It is defined as the operating range over which a receiver can linearly process signals, from a minimum level defined by noise or nonlinearities, to a maximum level defined by saturation or generation of nonlinearities. In modern systems, it is particularly important to preserve the capability to process small signals. If the dynamic range between stages of a receiver is not the same (i.e., the dynamic range of a following stage is less than the dynamic range of the preceding stage), as is often the case, matching of the dynamic range is required by compressing the dynamic range of the signal processed by the preceding stage to correspond to the dynamic range of the following stage.
Conventional receivers typically use one of three methods of dynamic range control (handling large signals): gain control, IF cancellation, or limiting. The first two methods are linear while the third is highly nonlinear. Gain control includes automatic gain control (AGC), in which the gain is varied as a function of the signal strength or some other control signal, and sensitivity time control (STC) in which the gain is varied as a function of range or time. IF cancellation or filtering is performed by delaying the received signal from one transmission and subtracting it from the received signal from the subsequent transmission(s). Both finite impulse response (FIR) and infinite impulse response (IIR) filters have been utilized. Limiting is performed by setting a maximum possible signal level in the receiver which is below the maximum amplitude which strong signals may have in previous stages.
Conventional methods of dynamic range control or compression are described, for example, in a textbook entitled Radar Handbook, by M. Skolnik, McGraw-Hill, 1970, section 5.6 (Gain Controlled Amplifiers) pp. 5-19 to 5-23.
The conventional methods have significant shortcomings in modern applications. In radar receivers, for example, a key requirement is to maintain detection capability of small signals, which represent desired target information, in the presence of large signals which represent clutter.
Devices based on AGC and STC reduce the gain to maintain large signals within the linear dynamic range of the following receiver stage. An example of an AGC system specifically designed for this purpose is disclosed in U.S. Pat. No. 3,130,400, entitled "PULSE AMPLITUDE COMPRESSION SYSTEM", issued Apr. 21, 1964, to C. Washburn. The main problem with AGC is that the gain reduction also applies to small signals, thereby resulting in an unacceptable reduction in detectability. STC suffers from the same drawback, and is restricted to relatively short ranges (as a function of the system design).
Typical IF cancellers and filters depend on fixed delay lines (e.g., acoustic lines), sampled delay lines (e.g., charge-coupled devices), or analog filters. The shortcomings of fixed delay lines include inflexibility or significant complications to achieve flexibility, and requirements for temperature control to maintain performance over temperature. Sampled delay lines are more flexible, but they also have shortcomings in the areas of dynamic range and temperature control. The impulse response of IIR filters is such that small targets can be masked by the response to large targets. In addition, limiters are not acceptable because the limiting process introduces nonlinearities and other signal anomalies which mask the desired small signals.